Routing protocols are used between routers to determine paths and maintain routing tables.Once the path is determined, a router can route a routed protocol.What Is a Routing Protocol?An autonomous system is a collection of networks under a common administrative domain.IGPs operate within an autonomous system.EGPs connect different autonomous systems.Autonomous Systems: Interior or Exterior Routing ProtocolsAdministrative Distance: Ranking RoutesClasses of Routing ProtocolsClassful Routing OverviewClassful routing protocols do not include the subnet mask with the route advertisement.Within the same network, consistency of the subnet masks is assumed.Summary routes are exchanged between foreign networks.Examples of classful routing protocols:RIP Version 1 (RIPv1)IGRPClassless Routing OverviewClassless routing protocols include the subnet mask with the route advertisement.Classless routing protocols support variable-length subnet masking (VLSM).Summary routes can be manually controlled within the network.Examples of classless routing protocols:RIP Version 2 (RIPv2)EIGRPOSPFIS-ISRouting Protocol Comparison ChartUsing the ip classless CommandVLAN-to-VLAN OverviewNetwork layer devices combine multiple broadcast domains.Dividing a Physical Interface into SubinterfacesPhysical interfaces can be divided into multiple subinterfaces.Routing Between VLANs with ISL TrunksRouting Between VLANs with 802.1Q TrunksSummaryRouting is the process by which an item gets from one location to another. In networking, a router is the device used to route traffic. Routers can forward packets over static routes or dynamic routes, based on the router configuration. Static routes can be important if the Cisco IOS software cannot build a route to a particular destination. Static routes are also useful for specifying a “gateway of last resort” to which all unroutable packets will be sent. A default route is a special type of static route used for situations when the route from a source to a destination is not known or when it is unfeasible for the routing table to store sufficient information about the route. Summary (Cont.)When the static routing configuration is complete, use the show ip route command to verify the configuration. Dynamic routing relies on a routing protocol to disseminate knowledge. A routing protocol defines the set of rules used by a router when it communicates with neighboring routers. The ip classless command prevents a router from dropping a packet destined for an unknown subnet. In a VLAN environment, frames are only switched between ports within the same broadcast domain so a Layer 3 device is required to enable inter-VLAN communication. Use ISL or 802.1q to enable trunking on a router’s subinterface.